1. Field of the Invention
The invention relates to a braking force control system for a vehicle, and more particularly to a braking force control system for a vehicle having a braking system capable of controlling braking force of each of front and rear wheels independently of one another as needed.
2. Description of Related Art
In a vehicle, such as an automobile, which has a braking system capable of controlling the braking force of each of the wheels independently of one another, when braking force of any of the wheels becomes excessively large, and an excessive large slip or skid occurs to the wheel, anti-skid control is performed in which the braking force of the wheel concerned is increased or reduced so as to reduce the slip.
When the vehicle is running on a so-called “split μ road” having different coefficients of friction on the left side and right side thereof, an excessively large slip is likely to occur to the wheel(s) on the side having the lower coefficient of friction; therefore, anti-skid control is performed only on one of the front wheels. As a result, a difference appears between the braking forces of the right and left front wheels, and an extra yawing moment is applied to the vehicle due to the difference in the braking force.
In order to reduce the extra yawing moment applied to the vehicle due to the anti-skid control when the vehicle is running on the split μ road, it has been proposed to suppress increase of the difference between the braking forces of the right and left front wheels when anti-skid control starts being performed on one of the front wheels. For example, when anti-skid control starts being performed on one of the front wheels, it is known to suppress increase of the braking force of the other front wheel laterally opposite to the above-indicated one front wheel. Also, according to a known technology as described in Japanese Patent Application Publication No. 9-249111 (JP 9-249111 A), if it is determined that the vehicle is running on a split μ road, increase of the braking force of the front wheel on the side of the road having the higher coefficient of friction is suppressed.
In the braking force control system of the related art as described above, when the brakes are applied while the vehicle is running on a split μ road, increase of the braking force of the front wheel on the side of the road having the higher coefficient of friction is suppressed, so that the difference between the braking forces of the right and left front wheels can be reduced. It is thus possible to reduce the extra yawing moment applied to the vehicle due to the difference between the braking forces of the right and left front wheels.
In the braking force control system of the related art, however, only the braking forces of the front wheels are controlled; therefore, under a situation where the force that can be generated by the rear wheels is reduced due to load shift toward the front of the vehicle due to braking, the braking force of the rear wheels is increased according to a driver's braking operation. Therefore, the rear wheels have a reduced capacity or capability of generating lateral force, and the lateral force of the rear wheels tends to be insufficient. In particular, this phenomenon is likely to occur when the brakes are applied to the vehicle during turning where the outer rear wheel is on the side of the road having the lower coefficient of friction.